Recirculating Steam Generator U-Bend Hydraulic Tube Expansion

ABSTRACT

A hydraulic expander for use in a tube containing a U-bend region is disclosed and claimed. The tool includes a sheath for transmitting hydraulic fluid therethrough to an inflatable bladder. The sheath is flexible, allowing it to be inserted through the U-bend region of the tube. The bladder is fluidly connected to the sheath and is thereby in fluid communication with a supply of hydraulic fluid. The length of the bladder and the metal end caps, along with the centering spacers on the expansion sheath, allows it to pass through the U-bend region of the tube such that it can be positioned inside the tube adjacent to a support outside the tube, such as an AVB. Hydraulic expansion of the bladder causes it to exert an outward force on the tube inner surface, causing it to plastically deform such that its outer surface comes into contact with the tube support, such as an AVB. This expansion will increase the friction between the tube and the AVB to minimize the risk of tube vibration damage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of U.S. Provisional Patent Application No. 61/725,541 filed on Nov. 13, 2012, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to tube expanders, and, more particularly, the present invention relates to hydraulic tube expanders with expansions performed from within a tube by a computer controlled process.

2. Description of the Related Art

While the present invention may be used in a variety of industries, the environment of a pressurized water reactor (PWR) nuclear power plant will be discussed herein for illustrative purposes. There are two major systems utilized in a PWR to convert the heat generated in the fuel into electrical power. In the primary system, primary coolant is circulated past the fuel rods where it absorbs the emitted heat. The heated fluid, which is in liquid form due to the elevated pressure of the primary loop, flows to the steam generators where heat is transferred to the secondary system. After leaving the steam generators, the primary coolant is pumped back to the core to complete the primary loop. In the secondary loop, heat is transferred to the secondary coolant, or, feedwater, from the primary side in the steam generators, producing steam. The steam is used to rotate a turbine, generating electricity. The secondary coolant leaves the turbine, passes through a condenser to remove residual heat, and the liquid feedwater is pumped back to the steam generators.

Inside of the steam generator, the hot reactor coolant flows inside of the many tubes and the feedwater flows around the outside of the tubes. There are two forms of steam generators: once-through steam generators, in which the tubes are straight, and U-bend steam generators, which are more common and in which the tubes contain a U-shaped bend.

In general, heat exchangers, and steam generators in the nuclear industry in particular, are susceptible to vibration-induced wear between the tubing and internal heat exchanger tube supports. This vibration is due to flow-induced forces acting on the tubing during normal operation, and can occur anywhere along the tube length, including the U-bend region of the tubes where flow is more predominantly cross-flow rather than axial. Typical steam generators include supports, called anti-vibration bars (AVBs), that are disposed between adjacent columns of the U-shaped tubes to prevent vibration and other movement of the tubes. However, unless the tubes are in physical contact with the AVBs, the AVBs will not prevent all movement of the tubes. Although precision machining and installation is employed, a small gap can exist between the AVBs and the U-bend portion of the tubes. Moreover, AVBs typically are not designed with specific features to prevent instability in the in-plane direction (that is, within the plane defined by the U-bend tube). Any spacing between the tubes and the AVBs can prevent the AVBs from providing adequate resistance to in-plane tube movement.

SUMMARY OF THE INVENTION

An apparatus and a process for alleviating spacing between the AVBs and the U-tubes are herein presented. The inventive hydraulic expander is configured for use in a tube containing a U-bend region, such as a U-bend tube of a steam generator in a pressurized water nuclear reactor power system. The tool includes a sheath for transmitting hydraulic fluid therethrough. The sheath is flexible, allowing it to bend and pass through the U-bend tubes. The sheath may include a number of layers, with an outer protective tube overlying an inner tube through which the hydraulic fluid is transmitted. A first end of the sheath is in fluid communication with a supply of pressurized hydraulic fluid, such as water.

The tool further includes an inflatable bladder that is fluidly coupled to a second end of the sheath. The bladder and metal end caps have a length that allows it to pass through the bend region of the tubes. The expansion bladder length, which is measured along a longitudinal axis of the bladder, may be approximately 0.5 in. to 1 in. The sheath transmits hydraulic fluid to the bladder. The bladder is firmed of an elastic material such as urethane and is adapted for radial expansion when hydraulically pressurized to exert outward force on the inner surface of the tube. In this manner, the expansion tool can exert a force to plastically deform the tube outward such that it is moved into contact with the AVBs. The friction imparted by this contact can prevent flow-induced vibration or other movement of the steam generator tubes.

The tool may also include an eddy current probe, such as an eddy current coil, that is connected to the sheath. Through use of the eddy current probe in known manner, the position of the AVB can be determined and the bladder aligned with the AVB to ensure the correct portion of the tube is expanded.

The tool may also include one or more spacers that are connected to the sheath. The spacers are configured to align the hydraulic expander within the tube to allow it to be inserted around the curved U-bend portion of the tube.

DESCRIPTION OF THE DRAWINGS

The present invention is described with reference to the accompanying drawings, which illustrate exemplary embodiments and in which like reference characters reference like elements. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive.

FIG. 1 shows a preferred U-tube expansion tool of the present invention.

FIG. 2 shows another view of the tool of FIG. 1.

FIG. 3 shows a close-up view of the expansion bladder and the end caps of the tool of FIG. 1.

FIG. 4 shows the expansion tool of FIG. 1 positioned within the tube, at an AVB location in the U-bend portion of the tube.

FIG. 5 shows a partial close-up view from FIG. 4 of the tool positioned within the tube before expansion. This view also shows the existing gaps between the tubes and AVBs in the as-fabricated (pre-expansion) condition.

FIG. 6 shows the cross-section of the tube after expansion. In this view the tube has moved into contact with the AVB, increasing the friction force between the tube and AVB and reducing the risk of tube vibration.

FIG. 7 shows a partial close-up view from FIG. 6 of the tool positioned within the tube after expansion. This view also shows the tubes in contact with the AVBs after the completion of the expansion process.

FIG. 8 shows a hydraulic expansion graph illustrating use of the expansion tool of FIG. 1, wherein the horizontal axis is an amount of piston displacement and the vertical axis is applied pressure.

FIG. 9 shows exemplary data curves for use with the expansion tool of FIG. 1, wherein the horizontal axis is an expansion parameter and the vertical axis is the resulting amount of tube outside diameter expansion.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 show a preferred U-tube expansion probe tool 1 of the present invention. The tool 1 includes an expansion sheath 10 and associated fittings, an expander bladder 11, undersized bullet style end fittings 12 and 15, and multiple centering spacers 13 along the expansion sheath length. FIG. 3 shows a close-up view of a preferred expansion bladder 11 and end caps 12, 15. An eddy current coil 14 may also be included. The sheath 10 preferably includes an outer tube and an inner tube. The outer tube serves as a protective sleeve for the inner tube, and is formed of a material such plastic tubing to resist impacts, cuts, etc. while having enough flexibility to bend through the U-bend tubes. The outer tube may also have graduated marks to aid in expansion bladder positioning. The inner tube preferably is flexible tubing formed of a material such as metal to handle the pressure imposed by the hydraulic fluid. A first end of the inner tube is fluidly connected to a supply of hydraulic fluid, such as pressurized water. The second end of the inner tube is fluidly connected to the bladder 11 to deliver hydraulic fluid thereto. The sheath 10, bladder 11, and end caps 12, 15 have a small diameter chosen such that the tool 1 fits within the heat exchanger tubes and is easily transmitted through the tubes. The sheath 10 is flexible such that it can be transmitted through the U-bend portion of the tubes. The plurality of spacers 13 are positioned on the outer tube of the sheath 10 to keep the tool 1 substantially centered within the steam generator tube for installation purposes. FIG. 4 shows the expansion tool 1 positioned within the tube 20 at AVB 25 location in the U-bend portion of the tube 20. FIG. 5 shows a close-up view of the tool 1 positioned within the unexpanded tube 20.

The bladder 11 is fluidly coupled to the sheath 10. When hydraulically charged with a predetermined amount of fluid pressure, the bladder 11 expands to exert a radially outward force onto the inner diameter of the tube in which it has been inserted. Preferably, the bladder 11 is formed of a urethane material. The bladder 11 and metal end caps 12, 15 are shortened such that it can fit within and be transmitted through the U-bend portions of the heat exchanger tubes. A preferred length of the bladder 11 is approximately 0.5 in. to 1 in. The reduced length and diameter allow access to the entire U-bend radius while allowing the proper expansion length to be made to improve the heat exchanger vibration conditions. Therefore an expansion can be placed at any location along the U-bend where tube vibration is expected. The assembly is capable of creating a range of diametric expansions, from 0.003 in. or smaller up to greater than 0.060 in. FIG. 6 shows the cross-section of the tube 20 after expansion. In this view the tube 20 has moved in contact with AVB25, increasing the friction force between the tube 20 and AVB 25 and reducing the risk of tube vibration. FIG. 7 shows a partial close-up view from FIG. 6 of the tool 1 positioned within the expanded tube 25.

The repair method uses a single expansion bladder 11 that is centered within the AVE (or, as an alternative, it can be positioned on both sides of the AVB to pin the AVB in position). Due to the length of this expander 11 and the height of the AVB, there is some overhang of the bladder 11 on either side of the AVB. This allows for better positioning of the expander within the U-tube since the expander can be mispositioned some amount without affecting the process.

The hydraulic expansion process is managed by a computer-controlled system. A preferred control system is described U.S. Pat. No. 5,606,792, the disclosure of which is incorporated herein by reference in its entirety. This system consists of computer controls, a hydraulic expansion box to provide the high pressure water, high pressure tubing, and the expander.

The process hydraulically expands ¾ in.×0.043 in. wall alloy 690 steam generator tubes in the U-bend region of the steam generators (although this process could also be used to expand other size tubes, such as 9/16 in. and ⅞ in. outside diameter tubing, or tubes in other types of heat exchangers which have U-bend tube vibration issues). This process is used to decrease the gap and increase the friction between the tubes and the AVBs in the U-bend portion of the steam generator, preventing in-plane motion of the U-tubes during plant operation. Laboratory qualification testing is performed to determine the expansion parameters that are input into the control computer to assure accurate and consistent tube-to-AVB tube expansions. This consistency is required in order to achieve the proper fit between the tube-and-AVB to improve the steam generator operation.

The expansion process is controlled by pressurizing the bladder 11. As the water is being injected, the displacement of the piston and the pressure within the hydraulic expansion box are monitored. The point at which the tube yields is determined by the computer control system by noting the change in slope of the pressure vs. piston displacement curve. Past this point, the piston continues to be displaced until an additional amount of water is added to the system (this preset volume of water is determined during testing). When the preset volume of water is reached, the control system automatically shuts off the pressurization process and depressurizes.

A hydraulic system calibration is performed after the equipment is setup and the expander is placed on the expansion sheath. The purpose of the system calibration is to eliminate the elasticity of the system (elastic deformation of the high pressure tubing and the compressibility of water) from the expansion process.

After the system calibration is complete, the expander 11 is ready for use in the heat exchanger. The tool 1 is inserted into the tube such that the bladder 11 and eddy current coil 14, if used, are positioned beyond the AVB location that is being expanded. The expansion sheath is withdrawn from the tube until the location of the center of the AVB is determined using the eddy current coil 14 in known manner. As an option, the expansion sheath is inserted into the tube to a known depth as indicated by graduated marks on the sheath or an appropriately positioned hardstop clamped on the sheath. The expansion sheath 10 is then withdrawn from the tube an additional predetermined amount such that the center of the bladder 11 is aligned with the center of the AVB that is being expanded. The expansion process then commences.

There are preset safeguards built into the control system to eliminate the possibility of damage to the tube or expansion system. These include a maximum pressure limit for the system and a maximum allowed piston travel. The system automatically shuts down and depressurizes if either of these limits are reached.

This equipment and process eliminates the need to work on top of the tube bundle, since all work is performed from inside the tube. This beneficially reduces radiological exposure to workers and facilitates performance of the work because work from the outside of the tubes may be difficult from an access standpoint.

An expansion parameter is employed during use of the inventive system to achieve an expected expansion size. This parameter represents a volume of water that will be added to the system after the tube yields. Parameter values are correlated with the amount of tube expansion by performing a series of expansions in a mockup setting. This mockup includes of multiple columns of tubes with multiple tubes per column. Thin bars between various tube columns simulate the AVBs. The expander is placed in the tube at the location of the AVBs and the computer-controlled expansion process begins. The computer control automatically senses when the tube yields (based on the decrease in pressure vs. piston displacement slope as illustrated in FIG. 8), and then the additional volume of water is added. When this additional volume of water is reached the expansion system is shut of and automatically depressurized. The expander is withdrawn from the tube and the tube is withdrawn from the mockup.

The mockup tube OD is measured at the AVB location (where the expansion was performed) and this expansion data, along with the volume of water that was input into the computer (the expansion parameter), is stored in a computer spreadsheet. Additional expansions are made, with varying e parameters and different tubing, to arrive at data curves. FIG. 9 shows exemplary data curves that might be generated in this manner, wherein the horizontal axis is the expansion parameter (SDD), which represents the additional volume of water added after the tube yields, and the vertical axis is the amount of tube outside diameter expansion. This information can then be used to set the field installation parameters and safety parameters (such as maximum expansion parameter and pressure values) to ensure that the tube is not over-expanded or the expander is not damaged during use.

While the preferred embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not of limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail (such as positioning the expander to a preset length dimension instead of using eddy current) can be made therein without departing from the spirit and scope of the invention. Thus the present invention should not be limited by the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Furthermore, while certain advantages of the invention have been described herein, it is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein. 

What is claimed is:
 1. A hydraulic expander for use in a tube containing a U-bend region, comprising: a sheath for transmitting hydraulic fluid therethrough, said sheath being flexible; and an inflatable bladder fluidly coupled to a first end of said sheath by end caps, said bladder and end caps having a longitudinal length allowing it to pass through the U-bend region of the tube, said bladder being adapted for radial expansion when hydraulically pressurized to exert an outward force on an inner surface of the tube.
 2. The hydraulic expander of claim 1, wherein said bladder has a longitudinal length of approximately 0.5 in. to 1 in.
 3. The hydraulic expander of claim 1, further comprising an eddy current coil coupled to said sheath.
 4. The hydraulic expander of claim 1, wherein said sheath includes graduated marks thereon for measuring an amount of insertion into the tube.
 5. The hydraulic expander of claim 1, further comprising a hardstop coupled to said sheath for limiting insertion of said sheath into the tube.
 6. The hydraulic expander of claim 1, further comprising at least one spacer coupled to said sheath, said spacer configured to center the hydraulic expander within the tube to facilitate expander insertion.
 7. The hydraulic expander of claim 1, wherein said sheath includes an outer protective tube and an inner tube in fluidly coupled to said bladder.
 8. A method of stabilizing a tube bundle of a heat exchanger having an anti-vibration bar positioned between adjacent tubes of the tube bundle, comprising: inserting a hydraulic expander within a tube of the tube bundle, said hydraulic expander including: a flexible sheath for transmitting hydraulic fluid therethrough, and an inflatable bladder fluidly coupled to a first end of said sheath by end caps, said bladder and end caps having a longitudinal length allowing it to pass through a U-bend region of the tube, said bladder being adapted for radial expansion when hydraulically pressurized to exert an outward force on an inner surface of the tube; positioning said hydraulic expander within the tube such that said bladder is located adjacent the anti-vibration bar; and pressurizing said bladder and thereby plastically deforming the tube such that an outer surface of the tube contacts the anti-vibration bar.
 9. The method of claim 8, wherein said pressurizing includes deforming the tube with enough force that the tube outer surface remains in contact with the anti-vibration bar after said bladder has been depressurized and said hydraulic expander has been withdrawn from the tube.
 10. The method of claim 8, wherein: said hydraulic expander further includes an eddy current coil coupled to said sheath; and said positioning includes using said eddy current coil to locate the anti-vibration bar so that said bladder can be positioned adjacent the anti-vibration bar.
 11. The method of claim 8, wherein: said sheath includes graduated marks thereon; and said positioning includes using said graduated marks to determine an amount of expander insertion into the tube such that said bladder can be positioned adjacent the anti-vibration bar.
 12. The method of claim 8, wherein: said sheath includes a hardstop coupled thereto; and said positioning includes using said hardstop to determine an amount of expander insertion into the tube such that said bladder can be positioned adjacent the anti-vibration bar 